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Post by thetank on Jun 27, 2008 14:13:30 GMT -5
Hello All!
I have recently purchased a fully functional '79 SR 500, here in Houston, TX. Although the bike works, it was in desperate need of some tuning. The previous owner, though nice and reasonable (sold it to me for $700, title in hand) fancied himself a bike restorer, even though he lived in a double wide trailer, did not have a garage, stored the bike outside, and new absolutely nothing about bikes. It was obvious from the method he used to crank the bike, to way it ran under throttle down the street, that he knew nothing about carb's.
I took the carb to work (at a Yamaha dealership) and the techs told me (checked with sudco) that you cannot even get rebuild parts for the original Mikuni pumper carb. Well, here's the cool part: I have a perfecty working Keihin 41mm flat slide off my 2003 YZ 450. It is actually smaller than the Mikuni, and I think with a little heat, and a little grease I could fit it up without any issues.
Has anyone else tried this, yet?
Thank you for your input.
Frank the Tank
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Post by andy on Jun 27, 2008 14:39:26 GMT -5
Hi Frank and welcome to the forum.First off,shop around there are definitely re-furb kits available for the stock carb vm34ss,a lot of owners swear by the stock item and provided it only needs a re-furb and theres no serious wear you should have no problem.On the other hand there are also a few members using flatslides,i'm not convinced about these as there seems a wider range of issues than the stock carb.On a personal note,I replaced my stocker with a 34vm non-pumper [new] and am perfectly happy with it,so simple to tune and change jets/needles without major stripdown or removal,cheap too.Finally,if it helps there will be plenty of people wanting to buy your stock carb if you change,thats how highly rated they are provided there not worn out.Check the forum out theres loads of info on fuelling.
Regards,
Andy
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Post by sjef on Jun 27, 2008 14:50:52 GMT -5
Hallo Frank,
have a fcr41 on my 600ccSR.imho ithink its a bit big for a standard sr engine it will do fine after some tuning.
best regards sjef
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Post by strick on Jun 27, 2008 21:06:28 GMT -5
I really think the 41mm flow is too high as well. Now, if you had the carb off of a YZ250F then I would be REAL curious the result. In time I will experiment with a Keihin, which I am intimately acquainted with, but I don't even have plates on my street tracker yet.
Keep in mine these old slow revving thumpers have about 60% of the HP of your YZ450f, even though they have 10% more cc's.
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Post by thetank on Jun 28, 2008 10:36:27 GMT -5
Hey Gang!
First off, I am seriously excited. . . I got it working!!! I left the carb standard setup:
41mm Keihin FCR, 42 pilot, 185 main. Took the metal "skin" off the intake boot,
greased the spickut on the carb, and put the hairdryer to the boot for about 120 seconds.
Applied my manhood, and BAM it was on!
I let it sit under a cooling fan for 5 min's, then removed the carb from the boot. I remounted the boot to the motor, then "manhooded" the carb back on.
She cranked in 6 or 7 kicks. Had to make a MAJOR counter clockwise adjustment on the idle nob to bring the RPM's down to idle. Only one issue, so far. Once the RPM's go up, they take a long time to come down, even with the clutch pulled in. Why is that?
Thank you for your input.
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Post by thetank on Jun 28, 2008 10:52:33 GMT -5
PS: I have ordered a JEMCO exhaust. I have ordered it with a street baffle installed, but with a separate race baffle, as I plan on doing some vintage events, and testing my various development efforts at local track days.
I hope this helps you with your input that I would much appreciate.
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Post by solo2racr on Jun 28, 2008 12:09:07 GMT -5
Hey Gang! First off, I am seriously excited. . . I got it working!!! I left the carb standard setup: 41mm Keihin FCR, 42 pilot, 185 main. Took the metal "skin" off the intake boot, greased the spickut on the carb, and put the hairdryer to the boot for about 120 seconds. Applied my manhood, and BAM it was on! I let it sit under a cooling fan for 5 min's, then removed the carb from the boot. I remounted the boot to the motor, then "manhooded" the carb back on. She cranked in 6 or 7 kicks. Had to make a MAJOR counter clockwise adjustment on the idle nob to bring the RPM's down to idle. Only one issue, so far. Once the RPM's go up, they take a long time to come down, even with the clutch pulled in. Why is that? Thank you for your input. With a 42 pilot jet, more than likely there is an air leak somewhere. The 41mm carb is way to big for all but the most modded engines. A 36mm on a stock (ish) engine is as big as you would want to go. The 36mm will also work well for the engine with quite a few other mods done to it. Given the choice between your stock carb and the 41mm.......go for the stock carb. |
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Post by strick on Jun 28, 2008 16:40:42 GMT -5
41mm refers to the size of the air orifice diameter - air box side. 41mm is too much air for the slow revving SR motor. As I said in my first post on this topic, a smaller Keihin would probalby work better than the stock carb. I just have ZERO idea on how small (in intake mm) that would be. If you can change the leak jet to a MUCH smaller one, I would be curious on the impact, but overall I don't think that carb will perform on the SR.
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Post by caferacercarl on Jul 3, 2008 3:38:20 GMT -5
My SR engines make the exact same output on the dyno as YZ450's, and I had a FCR41 on mine , parts for the stock carb? I have stocked every part for them for over 10 years and they are available, tell the spares guy's to try harder, the FCR's on the 450's are 39mm as well.....cheers everyone, keep hotting them up.
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Post by strick on Jul 3, 2008 9:54:24 GMT -5
My SR engines make the exact same output on the dyno as YZ450's, and I had a FCR41 on mine , parts for the stock carb? I have stocked every part for them for over 10 years and they are available, tell the spares guy's to try harder, the FCR's on the 450's are 39mm as well.....cheers everyone, keep hotting them up. I think it would take a turbocharger on my SR500 to produce the 51 rear wheel horsepower I have pumping out of my CRF450 (FCR41mm-Keihin). You say you HAD, what happened? I would LOVE to have a flat slide on my SR, but not something as large as a 41. --> Also thetank's bike sounds STOCK, which once again yields my original conclusion - it wont' perform properly. |
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Post by joneill4 on Jul 7, 2008 13:04:45 GMT -5
I believe that a bike is slow to return to idle, it means it is very lean. Solo may be right about the air leak.
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Post by caferacercarl on Jul 12, 2008 19:12:56 GMT -5
a guy hit me head on in my car and I have'nt ridden since, but have had a neck operation on three vertebrae and getting better after 7 years of pain....dissmantled my SR and sold it, YES you are right, stock SR and Keihin FCR, don't bother, [ would someone please tell deus this] my bike had a very heavily modifed engine, its all in the cam, ask stew ross he did his homework before the new project. but I kept the FCR, the billet trees and the 8 inch headlight. one day...... but the next bike will have to have handlebars due to medical reasons cheers..
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Post by thetank on Jul 21, 2008 14:37:31 GMT -5
OK, I have been unable to figure out the lean/rpm issue on this carb. I think I can sell off this 41mm carb, and pickup another carb for this bike. So, knowing that this motor is stock, what is the absolute best carb for it? Also, what is the best carb once I build it? (Hi Comp piston, upgraded cam, headwork done to maximize what the motor can do on pump gas- 93 octane.) Thanks in advance!
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Post by solo2racr on Jul 21, 2008 14:55:06 GMT -5
Probably the best compromise carb between stock and the mods you mentioned would be a 36mm Mikuni.
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Post by strick on Jul 22, 2008 9:38:32 GMT -5
As soon as I can find one for a decent price, I am going to buy a 37mm Keihin FCR from a CRF250 (R or X), or a YZ250F (WR250F). I will be sure to post back my results.
I have a stock cam and an overbore (511cc) 10:1 Wiseco piston, K & N filter, with header (and torque insert) and supertrapp.
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